140 Lord Oxmaritown on the cofistruction of 



then examined, and was found to be broken in several places. 

 A second metal was then cast precisely similar to the former 

 one and similarly treated, but the composition was a little 

 lowered. It also cracked, but not so much. A third was cast, 

 the composition being a little lower than that of the second ; 

 it also met with a similar fate. A fourth of a still lower compo- 

 sition was defective in casting, but did not crack ; the fifth 

 turned out well. The fourth and fifth were of the same compo- 

 sition. The metal has a slight tinge of yellow clearly percepti- 

 ble when compared with metal of the best composition. It 

 does not take so high a polish, and is more subject to tarnish. 

 The metal, however, was much higher than that of Sir Wil- 

 liam HerschePs twenty-foot telescope. 



Upon the whole, the result of the above-mentioned experi- 

 ments was by no means satisfactory. I found that I could not 

 cast a speculum of the moderate dimensions of fifteen inches, 

 without reducing the composition considerably below the high- 

 est standard. It was also quite evident that the composition 

 should be still lower for a metal three feet diameter ; such a 

 metal might indeed have been made of one-third the weight 

 which would otherwise have been necessary, by casting it like 

 the fifteen inch metal with a rim and ribs at the back ; but 

 still the defect in the quality of the metal would have re- 

 mained, which appeared to me to be a decisive objection to 

 the construction of such an instrument. 



After several fruitless attempts to combat this difficulty, 

 experiments were tried to ascertain whether it would be prac- 

 ticable to cast specula in different pieces, and to unite them 

 together by tinning the surfaces. This was found to be prac- 

 ticable, but it was abandoned for the following plan, which I 

 think was perfectly successful. 



An alloy of zinc and copper can be formed, which will ex- 

 pand and contract with changes of temperature more or less 

 than speculum metal, according to the proportion of the in- 

 gredients. Experiments were made, and it was found that cop- 

 per 2-1-1, and zinc 1, would give an alloy possessing the re- 

 quired property of giving expansions and contractions with a 

 change of temperature, not sensibly different from speculum 

 metal. This alloy is malleable, ductile, and easily worked. 



